Powder coating compositions for use in painting surfaces have become increasingly desirable in recent years for various reasons including those relating to ecology, health and safety. More particularly, such powder coating compositions have become desirable because they eliminate the solvents utilized in liquid paint systems such as are currently in wide use. The application and drying or curing of such liquid paint compositions, of course, requires that the solvent vehicle be volatilized, thus resulting in solvent escaping into the atmosphere and creating health and safety hazards as well as undesirable pollution problems. Powder coating compositions, on the other hand, are curable by heat in a manner such that little, if any, volatile material is given off to the surrounding environment.
While numerous powder paint compositions have been proposed, coatings formed from such compositions share various problems including low luster, poor color development, poor color matching and poor shading. The problems associated with such coatings are at least in part a result of the processes by which these powders are prepared. Prior art manufacturing methods for producing powder coating materials include ballmilling, Z-blade mixing and extrusion.
Ball milling is the simplest of these three methods. In the manufacture of epoxy based powder coatings, for example, all of the ingredients such as granulated epoxy resin, hardener or curing agent, pigments and additives are loaded into a ceramic lined ball mill. Ceramic grinding media of various size and shape are generally used to grind the materials for ten to fifteen hours to produce a blend. Other types of ball milling can be used to produce a similar blend in a shorter time. This method, however, does not achieve a good dispersion of pigments and other additives, thus resulting in powders which cure to form coatings of exceptionally low gloss and opacity. In addition, such a process does not allow for satisfactory color and shade matching of the coating.
In Z-blade mixing resin is first heated in a Z-blade mixer to at least its melting point and when molten all other ingredients except curing agent are added slowly. In general, dispersion requires about 6 hours after which the temperature of the Z-blade mixer is reduced and curing agent is added. As soon as the curing agent has been adequately mixed, the melt is cooled, pulverized and classified. The compositions formed by this process generally produce coatings having inadequate pigment dispersion and gloss requirements for top coat uses such as for automobiles. As was the case with the ball milling process it is also difficult to obtain adequate color and shade matching with this process. Also, since the process is a batch process, the mill has to be thoroughly cleaned after each run because of the build-up of heat sensitive material.
In the extrusion method, the resin being used is blended with all other ingredients in a high speed mixer and the mixture is then fed into a heated extruder. The extrudate is cooled, pulverized and classified. Although the coatings produced by this method are generally of better quality than those formed from powders made by the above processes, they still, like the others, suffer from lack of flexibility in providing color and shade matching. A still further disadvantage of this process stems from the fact that pigment is dispersed by extrusion, thus requiring slower extrusion rates and possible premature cross-linking due to longer dwell times at elevated temperatures. This premature cross-linking results in a powder which forms coatings having surface imperfections and reduced gloss.
Because of the obvious advantages of powder coatings from the standpoint of ecology, health and safety, it is desirable to produce powders which will form coatings which are commercially acceptable as surface coatings on articles of manufacture such as automobiles. Accordingly, it is the object of this invention to develop a process of preparing thermosetting powder compositions which will upon being heated to a suitable temperature upon a substrate form coatings which exhibit good luster, exterior durability, adhesion and impact strength. It is a further object of this invention to prepare a powder paint composition in such a manner that the powder may be color and shade matched against standards prior to application to a surface.